To provide telephone service throughout the country, an infrastructure of telephone lines has been established connecting telephone central offices to homes and businesses. Such telephone lines consist of twisted pairs of insulated copper wires of 19, 22, 24 and 26 gauge (AWG).
A twisted pair of copper wire possesses an impedance and a capacitance which increases with the length of the line. A signal transmitted over a telephone line is attenuated due to this impedance and capacitance. Therefore, the longer the transmission distance, the greater the signal degradation.
This infrastructure is suitable for the transmission of voice and data signals, which require a bandwidth of about 4 kilohertz, because the attenuation in a voice or data signal can be reduced by periodically inserting series inductance on the line, a practice termed loading. Therefore, voice and data signals can be transmitted between telephone handsets, computers, and facsimile machines for hundreds of thousands of feet on twisted pair.
Conventional telephone wire is unsatisfactory, however, for the transmission of video signals, which require a bandwidth of at least 4.2 megahertz, because of the degradation a video signal experiences in passing over a copper wire, particularly the portions of the video signal near the upper and lower ends of the frequency spectrum. Therefore, video signals can only be transmitted about 500 feet over twisted pair before loss of image.
Compression technology solves the distance problem associated with transmitting video signals over twisted pair. When a video signal is compressed, it can be transmitted for extended distances over a conventional telephone line; however, the video signal also loses clarity due to the image loss caused by the compression scheme. Therefore, compression has not proven itself as a viable alternative for applications requiring low image loss, such as in the delivery of television programming.
The transmission of video signals without image loss has heretofore required broadband transmission facilities, such as conventional coaxial cable and fiber optic cable, which are capable of moving large volumes of information for extended distances.
Conventional coaxial cable has been extensively used by the cable television industry in providing large amounts of television programming to homes throughout the country. While suitable for the one-way transmission of video signals, conventional coaxial cable is unsuitable for applications requiring two-way transmission, such as the provision of telephone service, home shopping, and movies on demand.
Because of its bi-directional capabilities, fiber optic cable is currently the facility of choice for the transmission of video, telephone service, and other interactive fare. However, while long distance companies have established extensive fiber optic networks throughout the country, most local exchange carriers have not replaced existing copper facilities with fiber because of the high cost of installation. For example, over 90% of the Bell companies' 2.7 million miles of telephone lines are conventional twisted pair. Therefore, because of the existing copper infrastructure already in place and the prohibitive cost of installing fiber optic lines, the provision of both interactive video and telephone service over the same facility has heretofore been limited to a small percentage of homes and businesses which have been connected to a central office with new installations of fiber optic cable.